Evaluation of World Trade Center dusts and girder coatings using a simulated precipitation leaching procedure

A subset of the
loose dust samples and samples of material coating girders collected
from around the World Trade Center was subjected to chemical leach tests
to examine potential release of metals from the dusts and beam coatings.
The USGS leach test is a modification of a test described in detail in
Hageman and Briggs (2000). This leach test and the one developed by
Hageman and Briggs (2000) are modifications of the US EPA 1312
(Synthetic Precipitation Leaching Procedure, or SPLP) method. The USGS
tests were originally designed as a screening method to quickly assess
potential metal release from mine wastes.

As applied to the materials from the World Trade Center, these leach
tests can be used to infer the potential for release of various metals,
anions, and cations from the dusts as a result of rainfall or
interactions with water used in fire fighting or street washing. The
test also provides an indication of metals that might be bioavailable
should the dusts be inhaled, ingested, or discharged into ecosystems.

For this leach test, deionized (DI) water (pH ~5.5) is used as the
extractant. Dust samples were leached at a 1:20 ratio (2.5 grams dust /
50 milliliters DI water). A representative subsample of each dust
sample was weighed on a balance. Each dust sample was then placed in a
125 milliliter (ml) high-density polyethylene (HDPE) bottle to which
50.0 ml DI water was added. Each sample was then shaken for 5 minutes.
Following shaking, the solution was allowed to settle for 5 minutes.
Leachate solutions were then filtered using plastic syringes and 0.45
micrometer pore size nitrocellulose membrane filters. Sub-samples of
the leachate were collected and preserved for further analysis.

The procedure uses deionized water as the extractant solution rather
than the synthetic acid rain used in the EPA 1312 method. It also uses
a 5 minute agitation rather than an 18-hour agitation. Hence, it is
possible that the concentrations of soluble metals measured with this
test would be less than those measured using EPA 1312 method. However,
it is likely that the leach procedure used in this study successfully
reveals the metals likely to be mobilized from the dusts and girder
coatings. A comparative study between the EPA Method 1312 (SPLP)
procedure and this simplified leach can be found in Hageman and Briggs
(2000).

The leachate samples were analyzed for major cations and trace metals
by Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS) following the
protocols outlined by Lamothe et al. (1999) and for anions by ion
chromatography. The elements measured by the chemical analyses are
those routinely measured by the USGS for studies of rocks, sediments,
soils, and environmental samples.

Leachate solutions for a subset of the dust samples were also
analyzed for dissolved mercury concentrations. For these samples, an
aliquot was filtered using a syringe and disposable 0.45 micrometer pore
size nitrocellulose filter, and then acidified and preserved by the
addition of a 1 percent sodium dichromate/concentrated nitric acid
solution in a ratio of 1:19 (one part sodium dichromate/nitric acid
solution to 19 parts water sample). Leachate samples were stored in
nitric acid-washed, flint glass bottles with Teflon lined lids. Samples
were then analyzed for mercury using a Lachat QuikChem Mercury Analyzer
with fluorescence detector. This method has a lower reporting limit of
5 part per trillion (ng/L).

Quality assurance-quality control information for the process and the
chemical analyses are available upon request.

Leach Figure 1. Plots showing the ranges (blue boxes) and
means (horizontal white bars) in pH and specific conductance (upper
plot), major cations and anions (middle plot), and trace elements (lower
plot) in leachate solutions from WTC dusts and girder coatings.
Elements for which one or more samples were below the analytical
detection limits are indicated by arrows extending downward from the
detection limit concentration. Abbreviations: mg/L = milligrams per
liter (approximately the same as parts per million); µg/L =
micrograms per liter (approximately the same as parts per billion);
mS/cm = milliSiemens per centimeter. For comparison, 1 milligram per
liter equals 1000 micrograms per liter, and 1 part per million equals
1000 parts per billion. Also, one mS/cm in specific conductance is
approximately equal to 1000 milligrams per liter dissolved solids.

Leach Figure 4.
Map of lower Manhattan showing variations (as
stacked bar charts) of metals and anions present in intermediate
concentrations in leachate solutions derived from the various dusts and
girder coating samples. Dust samples collected indoors are indicated by
the single hatch pattern and girder coating samples by the cross-hatch
pattern; all others are dust samples collected outdoors. Note changes
in scale of the concentration axis of the plots between this figure and
leach figures 2-6.

Leach Figure 5.
Map of lower Manhattan showing variations (as
stacked bar charts) in concentrations of predominant trace metals and
metalloids for leachate solutions derived from the various dusts and
girder coating samples. Dust samples collected indoors are indicated by
the single hatch pattern and girder coating samples by the cross-hatch
pattern; all others are dust samples collected outdoors. Note changes
in scale of the concentration axis of the plots between this figure and
leach figures 2-6.

Leach Figure 6.
Map of downtown Manhattan showing variations
(as stacked bar charts) in concentrations of less abundant trace metals
and metalloids for leachate solutions derived from the various dusts and
girder coating samples. Dust samples collected indoors are indicated by
the single hatch pattern and girder coating samples by the cross-hatch
pattern; all others are dust samples collected outdoors. Note changes
in scale of the concentration axis of the plots between this figure and
leach figures 2-5.

Results of the leach tests are summarized in Leach Table 1, and in Leach Figures 1-6.
The metal concentrations summarized in
Leach Table 1 may not represent truly dissolved material, because the
nitrocellulose filter (0.45 micrometer pore size) used to filter the
leachate fluids prior to analysis will not filter out metals present in
very small particles or colloids.

Interpretation

In general, the leachate solutions developed moderately alkaline to
alkaline pH values (8.2 - 11.8), and high specific conductances (1.31 -
3.41 milliSiemens/cm, indicating high dissolved solids). Alkalinities
of the leachate solutions were not measured due to insufficient sample
volume, but are by inference from the pH and specific conductances,
likely to be quite high. The leachate solutions are composed primarily
of sulfate, bicarbonate, carbonate, and calcium, with lesser
concentrations of the major cations sodium, potassium, and magnesium.

The alkaline pH of the leach solutions, coupled with the high
concentrations of calcium, carbonate, and sulfate, are consistent with
an origin resulting primarily from the dissolution of concrete, glass
fibers, gypsum, and other material in the dusts. The leach fluids with
the highest pH and highest specific conductance are from dust samples
collected indoors (including WTC01-20, collected indoors from the
gymnasium across West Street from the World Trade Center, and WTC01-36,
which was collected in a 30th-floor apartment in a building southwest of
the WTC). The higher specific conductances and pH values of indoor dust
samples indicate that the outdoor samples have already experienced some
leaching by rainfall and water used for fighting fires and street
cleaning between September 11 and the time that the samples were
collected. Leach solutions from the indoor dust samples also contain
slightly less sulfate, but greater calcium, than leach solutions from
several outdoor dust samples (Leach Figure 2). This suggests that
dissolution of concrete or glass fibers is greater in the indoor dusts
than in the outdoor dusts, and is another indication that the outdoor
dusts have already undergone some leaching by rainfall or wash waters.

Heavy metals and metalloids are present in low to quite high
concentrations in many of the leach solutions
Leach Table 1,
Leach Figure 1).
Mercury is present in generally low concentrations
in the leachate solutions from outdoor dust samples (near 10 nanograms
per liter, or parts per trillion). Mercury concentrations in leachate
solutions from indoor dust samples (as high as 130 nanograms per liter),
although low compared to concentrations of other metals in the leachate
solutions, are relatively high compared to mercury concentrations
measured in many types of environmental water samples. Arsenic, cobalt,
cadmium, thorium, and uranium are present in relatively low
concentrations in the leachate solutions (maximum concentrations of 3.3,
3.2, 1.6, 0.5, and 0.5 micrograms per liter, µg/L, respectively).
Lead, selenium, and vanadium are present in moderate concentrations
(maximum concentrations of 11.5, 10.5, and 16.1 µg/L,
respectively). Metals or metalloids present in relatively high
concentrations in the leachate solutions include (maximum concentrations
listed in parentheses): aluminum (702 µg/L), chromium (403
µg/L), antimony (74 µg/L), molybdenum (140 µg/L),
barium (62 µg/L), manganese (35 µg/L), copper (39
µg/L), and zinc (62 µg/L).

Of the various major and trace elements, aluminum is leached in
greatest amounts from the indoor dust samples relative to outdoor dust
samples. This indicates that the indoor dusts, in addition to having a
greater proportion of reactive concrete, also contain some sort of
reactive aluminum-bearing material. This material has presumably been
partly to largely leached from the outdoor dusts by rain water and wash
water.

Leachate solution from one of the beam coating samples (WTC01-09)
contain unusually high amounts of chromium (408 µg/L). As noted
in the SEM section, the mineralogy of this sample is generally similar
to those of the dust samples in overall mineralogy. However, the source
of the leachable chromium in the material is currently unknown.

The source of the metals and metalloids in the leach solutions is
unclear, but many components of the dust and debris are possible
sources, including particles from concrete, aggregate, gypsum wallboard,
glass fibers, construction steel, wiring, computer equipment, etc.

The results of the leach tests also show that metals are not leached
from the dusts and beam coating samples in proportion to their total
concentrations in the samples (compare concentrations in Leach Table 1
with those in Chemistry Table 1 from the previous section). For
example, chromium, molybdenum, and antimony are leached in relatively
high amounts from the samples, but occur in relatively low total
concentrations in the samples. In part, these trace elements are likely
being leached in greater proportions from the samples due to their
enhanced solubilities in alkaline solutions. It is also possible that
they are being leached more aggressively because they may occur in
materials that are more readily dissolved in alkaline solutions. In
contrast, iron, zinc and lead, which are relatively more abundant in the
samples (Chemistry Table 1), are leached in proportionally quite low
amounts from the samples (Leach Table 1). These metals are generally
less mobile in alkaline solutions, and may also occur in materials that
are not readily soluble in alkaline solutions.

Summary and potential environmental implications

Results of the leach tests indicate that the dusts released from the
WTC collapse, when exposed rainwater or wash water, likely produce
slightly alkaline to quite alkaline,
calcium-sodium-potassium-sulfate-bicarbonate-carbonate solutions. At
least some heavy metals and metalloids may be readily leached from the
dusts: aluminum, chromium, antimony, molybdenum, and barium are
generally leached in the greatest amounts, but other metals such as
zinc, copper, manganese, titanium, vanadium, lead and mercury are also
leached in measurable quantities. It is unclear if these heavy metals
and metalloids may be leached in sufficient quantities to be of
environmental or health concern. These results indicate that continued
EPA monitoring of runoff water quality is warranted. Continued rainfall
will likely continue to decrease the amounts of metals and alkalinity
that can be released from the outdoor dusts.

The results of the leach tests also indicate that cleanup of dusts
should be done with appropriate respiratory protection to prevent
possible inhalation of alkaline material with potentially bioavailable
heavy metals and metalloids. This is especially true for cleanup of
dusts from indoor localities that have not been exposed to rainfall.